1. Field of the Invention
The present invention relates to an ultrasound diagnosis apparatus, and, in particular, to a technique for generating a waveform of a transmission signal.
2. Description of the Related Art
In ultrasound diagnosis apparatuses, a plurality of transmission signals are supplied to a plurality of transducer elements, and an ultrasound is transmitted from the plurality of transducer elements to a living body. Each transmission signal is generated by a transmission circuit, and various circuits are known as a transmission circuit (for example, refer to JP 2001-245881 A and JP 2001-087263 A). For example, there are known a circuit in which a transmission pulse is generated as a transmission signal by a simple switching and a circuit in which a waveform of a positive polarity and a waveform of a negative polarity are generated and the waveforms are combined to generate a transmission signal. In recent years, a circuit is commercialized in which a transmission signal is generated by reading a digital signal representing a transmission waveform from a memory, converting the digital signal into an analog signal, and amplifying the analog signal by a liner amplifier.
In order to improve the resolution in a depth direction (axial resolution) in an image, a transmission pulse with a narrow pulse width (that is, pulse with a broad band) must be generated. In addition, in order to improve sensitivity, it is desirable to increase a voltage of the transmission signal. In particular, because a harmonic component (in particular, second order harmonic component) included in a reception signal is weaker compared to a fundamental component by approximately −20 dB, in a harmonic imaging mode (for example, refer to JP 2002-165796 A and U.S. Pat. No. 5,902,243), improvements in sensitivity of the harmonic component is strongly desired. In general, the second order harmonic component is proportional to the square of a transmission sound pressure, and, thus, it is desired to increase the transmission sound pressure, that is, the transmission voltage.
However, when a transmission pulse of a large amplitude is formed simply as a positive polarity pulse or a negative polarity pulse, a high voltage must be generated within the transmission circuit, and a circuit structure which can tolerate such a high voltage (such as a linear amplifier having a high voltage tolerance and a switching circuit having a high voltage tolerance) must be employed.
In order to form an image of the harmonic component, it is necessary to remove the fundamental component included in the reception signal and to extract only the harmonic component. As a method of realizing this process, there are known a pulse inversion method or a phase inversion method (for example, refer to JP 2002-165796 A and U.S. Pat. No. 5,902,243 as described above). In this method, a first transmission pulse and a second transmission pulse which is an inverted pulse of the first transmission pulse are alternately transmitted and a first reception signal and a second reception signal corresponding to the first and second transmission pulses, respectively, are added. U.S. Pat. No. 6,190,322 discloses a phase inversion method in which a first asymmetric ultrasonic pulse and a second asymmetric ultrasonic pulse which is an inverted pulse of the first asymmetric ultrasonic pulse are alternately transmitted. FIG. 4 of U.S. Pat. No. 6,190,322 shows a waveform of the first asymmetric ultrasonic pulse. The asymmetric ultrasonic pulse has an extended pressure peak and two shallow pressure peaks existing before and after the extended pressure peak. The waveform, however, does not represent an electric transmission signal. U.S. Pat. No. 6,190,322 fails to disclose the use of a meaningful negative offset, in particular, generation of an electric transmission signal including a negative offset which forms a basis of an impulse portion.